Participant followup rate can bias structural imaging measures in longitudinal studies

Loss of participants to followup (dropout) in longitudinal studies is inevitable and leads to great difficulty in interpretation of statistical results if dropout is correlated with a study outcome or exposure. In this article we test whether there is an additional problem that must be considered in longitudinal imaging studies, namely whether dropout has an impact on the function of FreeSurfer, a popular software pipeline used to estimate important structural brain metrics. We find that the number of acquisitions per individual does have an impact on the function of the FreeSurfer longitudinal pipeline, and can induce group differences in brain metrics.